This invention relates to small personal air samplers and particularly to their calibration for accurate readings of air or gas flow and sample volume.
Prior to the present invention, the calibration of personal air samplers involved in large part empirically obtained readings and sometimes unreliable manipulation by the user. Many of the calibrators employed bubble flowmeters.
An early patent to Gussman et al, U.S. Pat. No. 3,994,153, was directed to the calibration of the rotameter used to measure flow.
Conkle et al, in U.S. Pat. No. 4,569,235, maintain substantially constant air flow in an air sampler by monitoring flow rate change and using a signal representative of the flow rate change to manipulate the pump.
Padden et al, in U.S. Pat. Nos. 5,456,107 and 5,440,925, describe generating electrical signals representing flow rates and as a function of known volumes in one or more enclosures traversed by a piston. It is suggested that these signals may be used in data logging and report generation, but the device is not tied directly to the sampler pump for calibration in the manner of the present applicants.
Flow rates are calculated from the velocity of air in a sampler, as described by Buchan in U.S. Pat. No. 4,375,667. The flow rates are then integrated within the instrument over a sample period to provide an indication of the volume sampled, which the authors say enables them to obviate a calibration step. This disclosure employs a microprocessor and converters for processing the data obtained, but still may be said to calibrate only for a current sample and not as a standard for use over an extended period of time.
Ogden et al, in U.S. Pat. No. 5,551,311, uses a personal computer and describes a calibrating apparatus connected to it from the sampler. The system, however, does not utilize the data in the manner of applicants. See also Ogden et al U.S. Pat. No. 5,646,357.
Peck et al, in U.S. Pat. No. 5,107,713, describes a procedure which is manually repeated for several flow measurement readings; the user is prompted to enter the readings on a CPU, which further manipulates them based on an empirical compilation to establish a relationship between air flow, pulse width modulation, and pump motor RPM, using also a flow calibration meter. As described, the device is essentially self-calibrating, but does not tie the data generation to the calibrating device as applicants do.
The prior art approaches to calibration are not conducive to the calibration of a large number of personal air samplers within a short period of time. One may, for example, manually change the speed of the pump and then read a calibrator such as that described by Ogden et al above, marking down the data as it is collected. This process is susceptible to errors in setting the pump, recording the setting of the pump, and recording the reading of the calibrator. Over a period of time and a series of calibrations, errors are statistically likely.
Our invention includes a method of calibrating an air sampler at one desired flow rate or several wherein, for each desired flow rate, a plurality of air flow rates are averaged and analyzed, more or less automatically, for accuracy and stability. Where a range of flow rates is calibrated, a best-fit curve and/or a function representing it is generated. In either casexe2x80x94a single point or where a range of flow rates is calibratedxe2x80x94pump action is varied to deliver the desired flow.
Our technique permits the user to calibrate a large number of air samplers within a short period of time while minimizing the possibility of errors in entering data such as may happen with the Peck et al system mentioned above. In addition, our system is readily performed frequently and, perhaps more importantly, minimizes the possibility of introducing human error. Records may be maintained of the data obtained in and used for calibration.
We calibrate the air processing of a personal air sampler by (a) setting a desired air flow rate in the air sampler, (b) generating a signal representing a measured air flow rate through the air sampler set at the desired air flow rate, (c) recording the measured air flow rate in a data recorder (d) repeating steps (b) and (c) through at least one iteration (preferably a total of at least three repetitions), and (d) averaging at least two (preferably at least four) of the recordings so obtained. The average is compared to the setting of the desired air flow rate to determine the degree of difference, which is converted to a signal to be used as feedback for adjustment of voltage or power to the pump, or otherwise to adjust its output. The feedback signal may also be recorded and thereafter used to power the pump at the new level each time the tested air flow rate is desired. The pump and sampler are therefore calibrated for that particular desired air flow rate. Note that the difference of the measured flow from the desired flow, and the average difference from the setting, are both treated as positive numbers whether they are above or below the compared point.
Thus a straightforward calibration of a single desired air flow will result in the power supply to the pump being controlled to energize the pump to the degree necessary to achieve the desired air flow rate. As the typical sampler is battery-powered, the adjustment is preferably made to the voltage supplied to the pump. The new voltage will thereafter be used for that desired air flow rate until the sampler is calibrated again at that point.
But our procedure also guards against the possibility that an average of two or more measurements may be very close to the desired rate, while the individual measurements which make up the average are quite removed from the desired rate. A high total difference from the average indicates instability. Therefore we conduct the test for stability described below. Unlike the results of the averaging test, the results of the stability test are not used to adjust the power input to the pump. They are used to determine whether to discontinue use of the sampler.
To determine stability of the sampler, our procedure continues beyond the above recited sequence of steps with the steps of (e) determining and recording the absolute difference between each of the recordings and the average so obtained, (f) totaling the differences obtained, (g) expressing the total of differences in terms of a percentage of the average, and (h) comparing the percentage to a predetermined acceptable percentage. This percentage value may be used to decide whether to continue or discontinue the sampler in service, or the more extended procedure described further below may be followed.
While we speak throughout of personal air samplers, it should be understood that the same procedures and techniques may be used with respect to gases and gaseous media other than air, where the samplers are used to collect and/or measure contaminants, aerosols, microorganisms, and concentrations of various substances and other gases.
When we speak of xe2x80x9csignalsxe2x80x9d, we include not only the usual and typical electrical signals, but also pneumatic or other signals. They may be continuous (analog) or discrete (digital). Note also that the action of the pump may be varied by varying the voltage or power to it, by braking, actuation of a clutch, adjustment of the pump stroke, and/or any other practical means.